There are two basic techniques for performing analog-to-digital (A/D) conversion, open loop and feedback techniques. The open loop converter generates a digital code directly upon application of an input voltage; and is generally an asynchronous operation. The feedback technique generates a sequence of digital codes; reconverts these digital codes to an analog signal; and uses the reconverted signal to correct the input. For a more indepth discussion of these techniques, see Renschler, "Analog-to-Digital Conversion Techniques", Application Note AN-471, Motorola Inc. (1974).
Sigma-delta (.SIGMA.- .DELTA.) modulators have been known in the industry since the early l960s, but were used in limited capacities until more recently. With the advent of 16 and 32 bit processing, the .SIGMA.- .DELTA. modulators have found more use in audio and signal processing applications such as codersdecoders (codecs), modem front-ends, and Integrated System Digital Network (ISDN) applications. .SIGMA.- .DELTA. modulators have also found use in non-signal processing applications such as dc measurement.
The .SIGMA.- .DELTA. technique is attractive because it achieves high resolutior by precise timing instead of precisely matched on-chip components (resistors or capacitors). In addition, the expertise needed to produce thin film, laser trimmed analog components are difficult to obtain; whereas, high speed digital switching capability is commonplace in the semiconductor industry. This makes the .SIGMA.- .DELTA. technique, the technique of choice for many manufactures.
A basic .SIGMA.- .DELTA. ADC receives an analog signal which is summed with the inverse feedback of the digital output signal (after being reconverted to an analog signal) to provide an error signal. The error signal is then processed through a quantizer to provide a digital output bit stream. The digital output bit stream is also used to provide the feedback signal.